Data analysis for CO2-EOR in shale-oil reservoirs based on a laboratory database

Unconventional resources have played a significant role in changing oil industry plans recently. Shale formations in North America such as Bakken, Niobrara, and Eagle Ford have huge oil in place, 100-900 Billion barrels of recoverable oil in Bakken only. However, the primary oil recovery is still low as 5-10%. EOR methods are currently considered as a new concept in unconventional reservoirs due to the immature information about these plays. Injecting carbon dioxide (CO2) might be the most potential strategy to improve oil recovery in such complex plays (Alfarge et al., 2017a,b). Jin et al. (2016) conducted an experimental study to investigate improving oil recovery by CO2 injection in 21 natural preserved core samples from Bakken Petroleum System (BPS). Their experimental results indicated that CO2 injection has a significant potential to improve oil transportability in these Nano-pores formations. In this study, data analysis for feasibility of CO2-EOR has been conducted on 95 cases of natural preserved cores collected from different formations including 44 cases from Middle Bakken, 26 cases from Lower Bakken, 17 from Upper Bakken, 4 cases from Three Forks, and 4 cases from unknown formation/formations. The relationship between the improved oil recovery by the injected CO2 and 6 rock properties including porosity, permeability, mean pore throat radius, total organic carbon content (TOC), water saturation, and oil saturation has been separately determined and physically discussed. Furthermore, the relationships between the improved oil recovery obtained by the injected CO2 and 4 operating parameters including CO2 bath pressure, CO2 bath temperature, and core sample bulk size, and exposing time have been also separately investigated. A Proxy model to associate the functionality of the improved oil recovery by CO2 injection and these 10 parameters have been constructed. Moreover, statistical methods for Design of Experiments (DOE) have been used to rank the most important parameters affecting CO2-EOR performance in the microscale level (lab scale) of these unconventional reservoirs. This study provides an important set of lab-based data obtained from natural preserved cores to find out the applicability of CO2-EOR in these unconventional reservoirs. Also, this research demonstrates some key points which could help in understanding CO2-EOR mechanisms in shale plays since they are much complex and very different from conventional formations.